The present invention relates generally to the field of high-frequency multipole mass spectrometry and more particularly to a miniaturized mass spectrometer using a silicon chip field emitter array as the source of electrons for impact ionization of chemical species.
The number of applications for Quadrupole Mass Spectrometers (xe2x80x9cQMSxe2x80x9d) continues to increase. QMS instruments are used in the analysis of the environment for contaminants, medical testing and the development of new pharmaceuticals, energy research and biochemical analysis. Some QMS instruments are complex and relatively expensive research-grade instruments for biomedical and biochemical applications such as deducing the structure of proteins or the sequencing of DNA. On the opposite end of the spectrum, small, simple and inexpensive QMS devices are used as routine detectors for gas chromatography. Other types of QMS spectrometers are used by government agencies, for example in backpack portable instruments for in-situ analysis of hazardous chemicals in the environment, in mobile battlefield laboratories to warn of impending chemical or biological attack, or in enormous machines for the separation of atomic isotopes.
The ever-broadening range of applications of QMS spectrometers places ever-increasing demands on the performance of these devices. Unfortunately, existing technology has not always met current needs. New applications tend to require more specific, reliable mass analysis and more sensitive detection of ions having large mass-to-charge ratios which the current inventory of instruments cannot provide. However, this large, installed base of mass spectrometers represents a large and considerable capital investment in equipment and personnel that cannot readily be abandoned. Thus there is a desire to upgrade and improve the capabilities of existing instruments to meet the new demands.
Brief History of the Prior Art
The quadrupole mass filter is roughly 40 years old and is today widely used in a broad range of vacuum based instrumentation. Applications include sensitive leak detection, residual gas analysis, thermal desorption mass spectroscopy, molecular beam analysis, and detection in liquid and gas chromatography. Traditionally, these instruments have been very large laboratory devices owning principally to their need for very clean, very high rate vacuum systems, for high energy ionization sources and associated ion beam handling equipment, for sensitive detectors, and for heavy and sophisticated plumbing systems necessary to construct and house these instruments.
Since their development, multipole and in particular the QMS spectrometers have gained considerable scientific and commercial importance in many diverse fields ranging from chemical analysis to the establishment of highly precise atomic time standards. U.S. Pat. No. 2,939,952 entitled xe2x80x9cApparatus for Separating Charged Particles of Different Specific Chargesxe2x80x9d to Wolfgang Paul, et al., first describes the development of these devices. Many of the basic electrode configurations and anticipated uses for quadrupole mass filters are as shown or predicted in the ""952 patent. Today, most of the analytical mass spectrometers in use are of the quadrupole type. The proliferation and wide acceptance of QMS spectrometers can be attributed to their simplicity, reliability, and low cost compared to other types of mass spectrometers.
U.S. Pat. No. 5,464,975 provides a brief recitation regarding conventional prior art QMS systems. These systems are shown to consists of the following components (see FIG. 1): a sample inlet 1; an ion source 2 for converting the sample into charged species of certain mass-to-charge (m/z) ratios; a quadrupole mass filter 3 (also called a quadrupole mass analyzer) that preferentially passes one m/z ratio at a time; and a detector 4 to detect the abundance of the transmitted charged particles. By scanning the RF and DC voltages applied to the quadrupole mass filter, a mass spectrum can be generated, showing signal intensity, correlated to relative abundance (in arbitrary units), versus the m/z ratio (in Dalton units).
In most current mass spectrometers, the ionizer section comprises what is commonly known as a hot filament, using essentially vacuum tube technology, or a radioactive source for producing a stream of electrons or other high energy charged particles. It is these high energy species which serve to ionize the analyte stream thereby ionizing some portion of the material within the stream and which are subsequently separated in the quadrupole mass filter.
However, hot filaments and radioactive sources exhibit a number of disadvantages. Environmental and health and hygiene concerns limit the latter sources to essentially fixed laboratory facilities. Furthermore, high energy sources, especially those producing heavy particles, are intended to produce dissociation fragments of the parent molecule. This includes hot filament electrodes which exhibit a tendency to damage delicate molecules under analysis. Hot filaments also exhibit a property known as xe2x80x9coutgassingxe2x80x9d wherein the operation of the filament not only produces electrons but also xe2x80x9cboilsxe2x80x9d off metal atoms comprising the filament itself or absorbed or adsorbed species such as hydrogen, carbon monoxide/dioxide, and water, etc. This outgassing degrades the system cleanliness and reduces the high vacuum integrity of the system and, in turn, requires the use of large, rapid, and very expensive vacuum pumps in order to maintain operational pressures  less than 10xe2x88x929 Torr.
The instant application describes a modification to the QMS spectrometer which overcomes some of the shortcomings of prior art devices. The instant invention improves upon the performance of the QMS and suggests a route to miniaturization, portability, and instrument operation at higher pressures; features which are beyond the present state-of-the-art. The improvements embodied in the instant application apply equally well to the closely related monopole and multipole mass spectrometers. The improvement in QMS systems disclosed and described in the present work comprises the use of a Field Emitter Array (xe2x80x9cFEAxe2x80x9d) as the ionizing source for producing of electrons for impact ionization in a QMS. In particular, the FEA described herein is manufactured by means similar to those used for silicon integrated circuit (xe2x80x9cICxe2x80x9d) fabrication. Briefly, FEAs consist of a large number, (typically hundreds to tens of thousands) of sharp microscopic tips, each one of which is in close proximity to an electrode called a gate. Modest voltages (generally  less than 100 Volts) applied between the gates and the emission tips produce a high concentration of field lines on these tips. This, in turn, causes electron emission via tunneling through the work function barrier of the material comprising the tips into the vacuum.
The ionizer section of the instant invention, therefore, uses a cold cathode FEA in place of the typical hot filament electron gun source. This arrangement retains the advantages of high electron fluxes and fragmentation patterns to distinguish between species having the same mass (such as CO and N2) while avoiding deleterious effects of the hot filament source. Furthermore, miniaturization affords redundancy in the electron source beyond the customary two filaments used in conventional mass spectrometers.
Earlier attempts at finding new and different ionization sources have been addressed in U.S. Pat. No. 3,852,595 and U.S. Pat. No. 4,988,869, both to Aberth, and in U.S. Pat. No. 5,278,510 to Baptist.
The first of these references describes a high voltage electrode comprising an array of sharp projections on a conductive substrate. In this approach, Aberth suggests producing a high electric field by impressing an electrical potential of about 3600V across the substrate and a control grid spaced apart from and parallel to the plane of the array of projections. Ionization occurs when a gas or otherwise entrained sample is passed through the electric field. In the second approach, Aberth suggests using a field emission device to provide an electron current. This device, however, is similar to Aberth""s earlier electron source comprising a plate having a plurality of sharp protrusions on its surface. An electron stream is generated by impressing a potential between the plate and an electrically conductive grid parallel to and removed above the surface of the plate. This arrangement is also shown to be short range having a effective operational distance above the grid of no more than about a few tenths of centimeters. By way of contrast, the instant invention teaches an integrated FEA chip having emission tips each surrounded by an emission gate, rather than a remotely located grid, which produce electrons which are accelerated into an ionization chamber.
In the last of these references, U.S. Pat. No. 5,278,510 to Baptist, an ionization vacuum gauge is described which uses an electron source comprising a micropoint cathode and teaches that the technique for producing electrons via a field effect from such emissive micropoint is xe2x80x9c. . . fully known . . . xe2x80x9d and previously described in U.S. Patent Ser. Nos. 4,857,161 and 4,940,916.
Accordingly, the present invention provides a method and an apparatus for producing very high electron fluxes which are useful for causing collision-induced molecular dissociation.
Another object of this invention is to provide a means for easily retrofitting existing installed multipole mass spectrometers as well as adapting newly manufactured designs.
Yet another object of this invention is to provide a source of electron flux which is at once intense and essentially inert to the analyte species under investigation.
Another object of this invention is to provide a QMS wherein the ionizing electrons are generated by a cold cathode Field Emission Array.
Another object of this invention is to provide a QMS wherein the ionizing flux is remotely generated and subsequently directed into the analyte sample.
Another object of this invention is to provide a QMS capable of operating outside the present art of between about 10xe2x88x926 and 1019 Torr.
Another object of this invention is to provide a QMS capable of operating at internal pressure regimes below about 10xe2x88x923 Torr and above about 10xe2x88x9210 Torr.
Yet another object of this invention is to provide a QMS which is greatly smaller than existing devices.
Another object of this invention is to provide a mass spectrometer which generates no stray light during operation.
A further object of this invention is to provide a mass spectrometer having an electron-source with greater redundancy and therefore a spectrometer with greater reliability and a longer operational life.
Further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings. Other objects, advantages and novel features, and further scope of applicability of the invention will become apparent to those skilled in the art upon examination of the following drawings and description, or may be learned upon practicing the invention. The objects and advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims